This invention relates generally to power supply regulation and more particularly to secondary dynamic load detection with primary-side sensing and feedback.
Pulse width modulation (PWM) and pulse frequency modulation (PFM) are conventional technologies used for controlling switching power converters to achieve output power regulation. These conventional technologies may regulate the switching power convert to achieve constant voltage and constant current output regulation. A conventional flyback power converter includes a power stage for delivering electrical power from a power source to a load. A switch in the power stage electrically couples and decouples the load to and from the power source. A switch controller coupled to the switch controls the on-time and off-time of the switch, otherwise known as the duty cycle. The on-time and off-time of the switch may be modified based upon a feedback signal representing the output power, output voltage or output current by the controller to regulate the same. Input energy from the power source is stored in the gap of a transformer when the switch is on and is transferred to the load when the switch is off. Regulation can be accomplished by measuring the output current (or voltage) and feeding the measured output current (or voltage) back to a primary side controller, which can be used to modify the on-time and off-time of the switch of the switching power converter accordingly.
In order to improve cost performance and reduce over-all size, many commercially available isolated power supplies employ primary-only feedback and control. In primary-only feedback and control, secondary output and load conditions are detected by sensing primary side signals during each “ON” and “OFF” cycle and then controlled and regulated. This includes both constant voltage and constant current modes of operation.
However, many electronic devices require power supplies to provide a controlled and regulated power source over a wide range of operating conditions, which adds to the difficulty of primary-side sensing and control. Portable electronic devices such as smartphones and tablet computers are examples of such devices.
FIG. 1 illustrates an operating curve of an example switching power converter used to provide a controlled and regulated output to a load. Operating conditions presented to the switching power supply may occur while a load such as an electronic device is coupled to the power supply or when no-load is present. For example, in a Constant Voltage Mode (CVM) 101, the switching power supply supplies a regulated DC output of a fixed voltage within a certain tolerance range 104. CVM 101 operation generally indicates that the internal battery of the electronic device coupled to the power supply has been fully charged and the fixed voltage output of the power supply provides the operating power for the electronic device to be operated normally.
In a Constant Current Mode (CCM) 102, the power supply provides a fixed current output. CCM 102 operation generally indicates that the internal battery of the electronic device is not fully charged and the constant current output from the power supply allows for the efficient charging of the internal battery of the electronic device.
Lastly, in a No-Load condition 103, the electronic device is disconnected from the power supply. Under the No-Load condition 103, the switching power supply may maintain a regulated voltage output within the CVM range 104 in anticipation of an electronic device being reconnected to the power supply.
For convenience, end users oftentimes leave power supplies connected to the AC mains at times when no load is connected to the power supply output. Because the power supply should maintain a regulated output voltage even in no-load conditions, a dual-mode control methodology is commonly employed. During periods when a nominal load is coupled to the power supply, pulse width modulation is employed.
However, when a load approaches the no-load condition, it is difficult to maintain a duty-cycle low enough to maintain output regulation within the desired tolerance range 104. Accordingly, a pre-load, or dummy load may be added, however, the dummy load reduces operational efficiency during no-load conditions such that power consumption levels of the power supply are negatively impacted. Furthermore, because power supplies are oftentimes connected to the AC-mains for long periods of time when they are not connected to the electronic device, government and environmental agencies have placed maximum limits on the no-load power consumption.
One technique for control and regulation of low load or no-load conditions is for the controller to switch from PWM to PFM. Under no-load conditions in PFM, the rate of the pulses that turn on or off the power switch of the switching power converter is decreased significantly in order to maintain output voltage regulation, resulting in long periods of time between “ON” and “OFF” cycles of the switching power converter. This presents a significant challenge to primary-side sensing control schemes that rely on the “ON” and “OFF” cycles of the power switch to obtain a feedback signal. During the periods between “ON” and “OFF” cycle, the status of the output voltage may be unknown by the controller as no feedback signal is available. The lack of a feedback signal is especially concerning in the event the electronic device is reconnected to the power supply from a no-load condition. The reconnection of the electronic device presents a dynamic load change, and as a result of the long periods where the primary-side controller is unaware of the state of the secondary side output voltage, the power supply may ineffectively react to the load change. A poor dynamic load response in the above case would cause the output voltage to drop upon reconnection of the electronic device to the switching power converter. In some instances, this may cause the undesired affect of the amount of output voltage drop exceeding the regulation specifications. A dynamic load change may also present a significant challenge to switching power supplies that employ secondary output sensing.
FIG. 2 illustrates a conventional switching power converter 200 with a secondary-side feedback circuit 204. AC power is received from an AC power source (not shown) and is rectified to provide the regulated DC input voltage V_IN across input capacitor C0. The input power is stored in transformer T1 while the switch SW is turned on during the “ON” cycles because the diode D1 becomes reverse biased. The rectified AC input power is then transferred to the load L1 across the capacitor Co while the switch SW is turned off during the “OFF” cycles because the diode D1 becomes forward biased.
Secondary-side feedback circuit 204 comprises a compensation network of R1, R2 and RC circuit 209 including resistor R3 and capacitor C1, a reference device (not shown) for supplying reference voltage V_REF and a driver (e.g., an operational amplifier) 207 for driving an opto-isolator 205. The compensation network typically comprises an RC network 209 for closed-loop control and regulation.
The primary side of opto-isolator 205 is coupled to the feedback pin FB of controller 201 and conducts based on the voltage (nominally high) on the secondary side of the opto-isolator 205. Driver 207 provides the feedback voltage to the FB pin of controller 201 through the opto-isolator 205 while the regulated output voltage 203 reaches a regulation threshold at node N1 as compared to V_REF. As switch 202 turns ‘ON’ and ‘OFF’, the controller 201 utilizes the feedback on the feedback pin FB to determine a duty cycle of the power converter for controlling the switch 202.
It would seem natural to assume that since secondary feedback circuit 204 is directly coupled to regulated output 203, that the illustrated secondary-side feedback circuit 204 should be uniquely suited to respond to dynamic load changes. However, since secondary feedback circuit 204 is an integral part of the feedback and control loop, care must be taken to insure loop stability and prevent oscillation. Consequently, in designing a typical control feedback loop, there is a trade-off between loop stability and response time bandwidth.
FIGS. 3A, 3B, and 3C illustrate example waveforms corresponding to power converter 200 with secondary-side feedback circuit 204. FIG. 3A illustrates a time t—1 where a load LOAD placed on power supply 200 output 203 dynamically changes, drawing 100% rated output current from a previous 0% rated output current. The increase in current draw at the output 203 causes a decrease in the voltage V_OUT across the load LOAD at a rate largely based on the output current and output filter capacitance C2. As illustrated by FIG. 3B, to minimize the magnitude of the voltage drop (V_Drop1) of regulated output 203 and recovery time of the regulated output voltage V_OUT, the bandwidth of secondary feedback circuit 204 may be maximized. However, if the bandwidth is too wide, the control loop of power supply 200 may become instable in certain operating conditions. Instability of the control loop may result in adverse regulated output 203 conditions such as V_out oscillations. Conversely, as illustrated by FIG. 3C, when the bandwidth of secondary feedback circuit 204 is reduced to maintain control loop stability, the magnitude of the voltage drop (V_Drop) of regulated output 203 increases, causing the power supply 200 to approach or exceed the regulation specification. Moreover, the secondary-feedback circuit 204 and opto-isolator 205 are normally in the “ON” state in order to achieve feedback and control regulation. These devices may consume a substantial amount of power, even under the no-load condition.